U.S. patent number 7,629,194 [Application Number 11/845,602] was granted by the patent office on 2009-12-08 for metal contact rf mems single pole double throw latching switch.
This patent grant is currently assigned to HRL Laboratories, LLC. Invention is credited to Hui-Pin Hsu, Tsung-Yuan Hsu, James H. Schaffner, Adele E. Schmitz.
United States Patent |
7,629,194 |
Schaffner , et al. |
December 8, 2009 |
Metal contact RF MEMS single pole double throw latching switch
Abstract
Apparatus for a micro-electro-mechanical switch that provides
single pole, double throw switching action. The switch has two
input lines and two output lines. The switch has a seesaw
cantilever arm with contacts at each end that electrically connect
the input lines with the output lines. The cantilever arm is
latched into position by frictional forces between structures on
the cantilever arm and structures on the substrate in which the
cantilever arm is disposed. The state of the switch is changed by
applying an electrostatic force at one end of the cantilever arm to
overcome the mechanical force holding the other end of the
cantilever arm in place.
Inventors: |
Schaffner; James H.
(Chatsworth, CA), Hsu; Tsung-Yuan (Westlake Village, CA),
Schmitz; Adele E. (Newbury Park, CA), Hsu; Hui-Pin
(Northridge, CA) |
Assignee: |
HRL Laboratories, LLC (Malibu,
CA)
|
Family
ID: |
38562146 |
Appl.
No.: |
11/845,602 |
Filed: |
August 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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11006426 |
Dec 6, 2004 |
7280015 |
|
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Current U.S.
Class: |
438/52 |
Current CPC
Class: |
H01H
59/0009 (20130101); H01H 2059/0054 (20130101); H01H
2001/0047 (20130101) |
Current International
Class: |
H01L
21/00 (20060101) |
Field of
Search: |
;438/52 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Jensen, B.D., et al., "Design Optimization of Fully-Complaint
Bistant Micro-Mechanism," Proceedings of 2001 ASME International
Mechanical Engineering Congress and Exposition, New York, New York,
pp. 1-7 (Nov. 11-16, 2001). cited by other .
Sun, Xi-Qing, et al., "A Bistable Microrelay Based on Two-Segment
Multimorph Cantilever Actuators," The Eleventh Annual International
Workshop on Micro-electro Mechanical Systems, pp. 154-159 (Jan.
25-29, 1998). cited by other.
|
Primary Examiner: Nguyen; Ha Tran T
Assistant Examiner: Campbell; Shaun
Attorney, Agent or Firm: Ladas & Parry
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. application Ser. No.
11/006,426, filed on Dec. 6, 2004, now U.S. Pat. No. 7,280,015, the
disclosure of which is incorporated herein by reference.
Claims
What is claimed is:
1. A method of fabricating a switch comprising: providing a
substrate; depositing first conductive material on the substrate to
form an anchor pad, a first bias substrate electrode, and a second
bias substrate electrode; depositing a support layer on the first
conductive material and the substrate so that an upper contour of
the support layer follows a first contour of the first bias
substrate electrode; forming an anchor receptacle in the support
layer to expose the anchor pad; depositing a first beam structural
layer on the support layer, the first beam structural layer having
a first arm projecting in a first direction from the anchor
receptacle and having a second arm projecting in a second direction
from the anchor receptacle and a bottom contour in the first arm of
the first beam structural layer having a form to provide a means
for latching the first bias substrate electrode to the first beam
structural layer; forming a first contact receptacle in the first
arm at or near an end of the first arm; forming a second contact
receptacle in the second arm at or near an end of the second arm;
depositing second conductive material on a portion of the first
arm, on a portion of the second arm, in the anchor receptacle, and
in the first and second contact receptacles; depositing a second
beam structural layer on the first beam structural layer and on the
second conductive material; and removing the support layer.
2. The method according to claim 1, wherein depositing the first
conductive material further comprises depositing conductive
material to form a first input line, a first output line, a second
input line, and a second output line.
3. The method according to claim 1, wherein the first contact
material comprises a 900 angstrom layer of gold germanium, a 100
angstrom layer of nickel, and a 1500 angstrom layer of gold.
4. The method according to claim 1, wherein forming the first
contact receptacle and forming the second contact receptacle
comprises etching the first beam structural layer to form openings
in the first beam structural layer and partially etching a portion
of the support layer in the regions defined by the openings in the
first beam structural layer.
5. The method according to claim 1, wherein the second conductive
material comprises a 200 angstrom layer of titanium and a 1000
angstrom layer of gold.
6. The method according to claim 1, wherein the support layer
comprises silicon dioxide.
7. The method according to claim 6, wherein removing the support
layer comprises wet etching with hydrofluoric acid.
8. The method according to claim 1, wherein the first beam
structural layer and/or the second beam structural layer comprise
silicon nitride.
9. The method according to claim 1, wherein depositing the support
layer comprises sputter depositing silicon dioxide using plasma
enhanced chemical vapor deposition.
10. The method according to claim 1, wherein the support layer is 2
microns thick.
11. The method according to claim 1, wherein depositing second
conductive material comprises sputter deposition of 200 angstrom
layer of titanium followed by a deposition of a 1000 angstrom layer
of gold.
12. The method according to claim 1, wherein the first arm of the
first beam structural layer is formed so that a friction between
the first beam structural layer and the first bias substrate
electrode may be overcome.
13. The method according to claim 1, wherein: the support layer on
the first conductive material and the substrate has an upper
contour that follows a second contour of the second bias substrate
electrode; and the bottom contour in the second arm of the first
beam structural layer has a form to provide a means for latching
the second bias substrate electrode to the first beam structural
layer.
14. The method according to claim 13, wherein the second arm of the
first beam structural layer is formed so that a friction between
the first beam structural layer and the second bias substrate
electrode may be overcome.
15. The method according to claim 1, wherein: the first beam
structural layer is deposited on the anchor pad through the anchor
receptacle; and the first and the second arms are cantilevered at
the anchor pad.
Description
BACKGROUND
1. Technical Field
The present invention relates generally to switches. More
particularly, it relates to microfabricated electromechanical
switches having a single pole double throw configuration with the
ability to latch.
2. Description of Related Art
Switch networks are found in many systems applications. For
example, in satellite systems, switch networks are essential for
routing matrices and redundancy systems. Future satellite systems
will not only require larger switch routing networks, but also
increased functionality for network-centric operations. These new
capabilities will include spacecraft reconfiguration for beam
switching, beam shaping, and frequency agility. Thus, it is
expected that satellites will require an increasing number of
switches in their payloads.
In many cases, these switches need to be latching, that is, once
they are actuated they will remain in a desired state even after
the actuation energy source is removed. Some of the applications
where latching switches are important are ultra-reliable networks
where power interruptions could create a problem, such as satellite
or Unmanned Air Vehicles, or networks where supplied power is
limited, like in small mobile platforms that run on batteries.
Current latching switch technology typically relies on magnetic or
motor drives to change switch states. These switches, typically
fabricated using coaxial conductors or metallic waveguides,
generally work very well. However, most of the applications listed
above would benefit from size and weight reduction since the
mechanical latching switches currently in use tend to be larger and
heavier than desired. Semiconductor switches, such as made using
PIN diodes and FET switches, are small, but they typically cannot
latch in multiple states without a constant energy source.
Radio Frequency (RF) Micro Electro-Mechanical System (MEMS)
switches are known in the art to have small size and weight and are
also known to provide desirable performance in the radio frequency
and microwave spectrums. Several types of MEMS switches are
well-known in the art. For example, U.S. Pat. No. 5,121,089 issued
Jun. 9, 1992 to Larson discloses a microwave MEMS switch. The
Larson MEMS switch utilizes an armature design. One end of a metal
armature is affixed to an output line, and the other end of the
armature rests above an input line. The armature is electrically
isolated from the input line when the switch is in an open
position. When a voltage is applied to an electrode below the
armature, the armature is pulled downward and contacts the input
line. This creates a conducting path between the input line and the
output line through the metal armature. This switch requires a
constant voltage to maintain the switch in a closed state.
As another example, U.S. Pat. No. 6,046,659 of Loo et al. discloses
methods for the design and fabrication of non-latching single pole
single throw MEMS switches. U.S. Pat. No. 6,046,659 is incorporated
herein by reference in its entirety. FIG. 1 shows a top view of a
MEMS switch 10 according to Loo et al., which provides single pole
single throw switching between an input line 20 and an output line
18 when electrically actuated with a DC voltage.
FIGS. 2A and 2B are side-elevational views of the MEMS switch 10.
FIG. 2A shows the switch 10 in the open position and FIG. 2B shows
the switch 10 in the closed position. Beam structural material 26
is connected to a substrate 14 through a fixed anchor via 32. A
suspended armature bias electrode 30 is nested within the
structural material 26 and electrically accessed through a bias
line 38 at an armature bias pad 34. A conducting transmission line
28 is at the free end of the beam structural layer 26 and is
electrically isolated from the suspended armature bias electrode 30
by the dielectric structural layer 26. Contact dimples 24 of the
transmission line 28 extend through and below the structural layer
26 and define the areas of metal contact to the input and output
lines 20 and 18, respectively. A substrate bias electrode 22 is
below a suspended armature bias electrode 30 on the surface of the
substrate 14. When a voltage is applied between the suspended
armature bias electrode 30 and the substrate bias electrode 22, an
electrostatic attractive force will pull the suspended armature
bias electrode 30 as well as the attached armature 16 towards the
substrate bias electrode 22. The contact dimples 24 touch the input
line 20 and the output line 18, so the conducting transmission line
28 bridges the gap between the input line 20 and the output line
18, thereby closing the MEM switch.
Loo et al. generally describe a surface micromachined device. That
is, layers are deposited on top of a substrate, and then one or
more of the layers is etched away to release the moving parts of
the switch 10. As described in Loo et al., the parts of the switch
generally comprise gold (or gold alloys) for the switch contacts,
silicon dioxide for the one or more layers etched away (i.e., the
sacrificial layers), and silicon nitride for the beam structural
layer. However, as discussed in additional detail below, switches
fabricated according to Loo et al. may exhibit some problems.
The switches fabricated according to Loo et al. are typically
fabricated with one layer deposited on the next. With such
fabrication, any pattern of one layer may get transferred to each
subsequent layer. The dimensions of the switch dielectric and metal
layers are typically thin enough that the transferred copies of the
initial metal layer pattern (for example, the pattern of the
substrate bias electrode 22) appear even at the top nitride layer
of the dielectric structural layer 26. Therefore, as layers of
SiO.sub.2 and Si.sub.3N.sub.4 are deposited on top of the bottom
metal layer, these dielectric layers may wrap around the bottom
metal structures, in particular, the substrate bias electrode 22.
In some cases, after the sacrificial silicon dioxide was etched
away, the remaining silicon nitride formed a lid that covered the
substrate bias electrode 22 when the switch 10 was closed.
The formation of the silicon nitride "lid" is shown in FIG. 5,
which illustrates the dielectric structural layer 26 wrapping
around the bias electrode 22 disposed on the substrate 14. Because
of the tightness of the fit of this nitride "lid" over the bottom
electrode, there may be great deal of friction between the lid and
the substrate bias electrode 22 when the switch 10 is opened and
closed. The friction of the lid may depend upon post-processing
used to etch away the sacrificial layer. The lid may be made to fit
more loosely over the substrate bias electrode 22 by etching
longer, so that some of the silicon nitride is etched away.
However, in some cases, the switch 10 would close upon actuation
and not open upon the removal of the actuating voltage. Therefore,
as indicated above, control of the design of the switch and the
processes used to fabricate the switch may be required to avoid the
friction problems in the prior art switch according to Loo et
al.
An example of a latching micro switch is described in U.S. Pat. No.
6,496,612 issued Dec. 17, 2002 to Ruan et al. Ruan et al. describe
a switch having a cantilever to switch between an open state and a
closed state. To operate as a latching switch, a permanent magnet
is used to maintain the cantilever in an open state or a closed
state. However, the use of a permanent magnet may result in a
switch that is bigger and/or heavier than desired.
Another example of a latching switch is described by Xi-Qing Sun,
K. R. Farmer and W. N. Carr in "A Bistable Micro Relay Based on
Two-Segment Multimorph Cantilever Actuators," The Eleventh Annual
International Workshop on Micto-electro Mechanical Systems, 1998,
MEMS 98 Proceedings, Jan. 25-29, 1998, pp. 154-159. Sun et al.
describe a latching switch mechanism that uses two metals to create
stresses in opposite directions along a cantilever beam. RF
contacts can be moved by controlling the stress on the two segments
electrostatically to lengthen or shorten the length of the
cantilever along the substrate so that the contact can be moved
from one RF line to another. The fabrication of the switch
disclosed by Sun et al. may be complicated since two different
metals are required. Further, the switch disclosed by Sun et al.
requires two independent control voltages to move the switch.
Still another example of a single pole double throw switch is
described in U.S. Pat. No. 6,440,767 B1, issued Aug. 27, 2002 to
Loo et al. This switch is similar to that described above in U.S.
Pat. No. 6,046,659, except that two armatures are used to provide
the single pole double throw switching action. As such, the switch
may exhibit the same problems described above in regard to the
switch disclosed in U.S. Pat. No. 6,046,659.
Therefore, there is a need in the art for a small, lightweight
latching switch that does not require an external voltage or
magnetic source to stay latched in a selected state.
SUMMARY
Embodiments of the present invention provide for a method and
apparatus for switching that is bistable. An embodiment of the
present invention comprises a SPDT RF MEMS metal contact switch
that is bistable. According to embodiments of the present
invention, a non-planar processing technique may be used to provide
a switch that sticks in one of two positions when electrostatically
actuated. Embodiments of the present invention employ a frictional
latching mechanism that is provided by portions of a switch
cantilever beam that fit snugly around parts of a metal layer
deposited beneath the cantilever beam. Embodiments of the present
invention also employ a seesaw switch structure with two actuation
electrodes that pull down one side of the cantilever beam or the
other.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages will become more apparent
from a detailed consideration of the invention when taken in
conjunction with the drawings described below. However, this
invention may be embodied in many different forms and should not be
construed as limited to the embodiments depicted in the drawings or
described below. Further, the dimensions of certain elements shown
in the accompanying drawings may be exaggerated to more clearly
show details. The present invention should not be construed as
being limited to the dimensional relations shown in the drawings,
nor should the individual elements shown in the drawings be
construed to be limited to the dimensions shown.
FIG. 1 (prior art) is a top view of a prior art RF MEMS switch.
FIG. 2A (prior art) shows a cross-sectional view of the switch in
FIG. 1 in an open position.
FIG. 2B (prior art) shows a cross-sectional view of the switch in
FIG. 1 in a closed position.
FIG. 3 shows a top view of a switch according to an embodiment of
the present invention.
FIG. 4 shows a side view of the switch shown in FIG. 3.
FIG. 4A shows a close up view of a portion of the switch shown in
FIG. 4.
FIG. 5 shows the formation of a lid over metal deposited on a
substrate.
FIGS. 6A-6F show the fabrication of a switch according to an
embodiment of the present invention.
DETAILED DESCRIPTION
It should be appreciated that the particular embodiments shown and
described herein are examples of the invention and are not intended
to otherwise limit the scope of the present invention in any way.
Indeed, for the sake of brevity, conventional electronics,
manufacturing, MEMS technologies and other functional aspects of
the systems (and components of the individual operating components
of the systems) may not be described in detail herein. Furthermore,
for purposes of brevity, embodiments of the invention are
frequently described herein as pertaining to a micro
electro-mechanical switch for use in electrical or electronic
systems. It should be appreciated that many other manufacturing
techniques could be used to create the embodiments described
herein. Further, the embodiments according to the present invention
would be suitable for application in electrical systems, optical
systems, consumer electronics, industrial electronics, wireless
systems, space applications, or any other application. Moreover, it
should be understood that the spatial descriptions (e.g. "above",
"below", "up"? "down", etc.) made herein are for purposes of
illustration only, and that embodiments of the present invention
may be spatially arranged in any orientation or manner.
As described above and shown in FIG. 5, the deposition of
sacrificial silicon dioxide and silicon nitride over a metal layer
disposed on a substrate may cause the pattern of the metal layer to
appear in the silicon nitride layer. As additionally explained
above, this may cause the formation of a "lid" in the silicon
nitride layer that causes a cantilever arm in which the lid is
formed to stick to the underlying metal layer. As described above,
such a feature is generally considered a problem with prior art
devices. However, embodiments of the present invention may be
designed to rely upon this feature to achieve a desired latching
effect.
Embodiments of the present invention use a lid formed in a
cantilever arm to hold the switch in position even after the
actuation voltage is released. According to embodiments of the
present invention, the frictional forces will need to be larger
than the spring forces in the cantilever beam which want to restore
the cantilever to its equilibrium position. The required relatively
large frictional forces may be achieved by a lid created during
processing.
A top view of a switch 100 according to an embodiment of the
present invention is shown in FIG. 3. FIG. 3 shows a first input
line 126, a first output line 124, a second input line 136, and a
second output line 134 disposed on a substrate. The switching
function is provided by a seesaw cantilever structure 110
comprising a first cantilever arm 120 and a second cantilever arm
130. The switch 110 is actuated by pivoting the cantilever
structure at a cantilever anchor 117 (shown in FIG. 4). Voltages
are applied at a first bias pad 123 and/or a second bias pad 133 to
cause the cantilever structure to move in a first direction of a
second direction due to electrostatic attraction. A common pad 113
provides a return path or ground path.
FIG. 4 shows a side view of the switch 100 shown in FIG. 3 and
illustrates additional features of the switch 100. As shown in FIG.
4, the cantilever structure 110 comprises a first beam structural
layer 116, an armature electrode layer 112, and a second beam
structural layer 114. Preferably, the first beam structural layer
116 and the second beam structural layer 114 comprise silicon
nitride, but other materials such as polymer materials may be used.
The cantilever structure 110 is anchored to the substrate 105 by
the cantilever anchor 117, which comprises portions of the first
beam structural layer 116 and the armature electrode layer 112.
Preferably, the cantilever anchor 117 is flexible to facilitate the
latching and unlatching of the switch, as is described in
additional detail below. An anchor pad 111 provides an electrical
connection between the common pad 113 and the armature electrode
layer 112 at the cantilever anchor 117.
The first cantilever arm 120 and the second cantilever arm 130
project from the cantilever anchor 117. The first cantilever arm
120 is disposed over a first substrate bias electrode 122. The
first cantilever arm 120 also has a first contact 128 that bridges
a gap between the first input line 126 and the first output line
124. When the first cantilever arm 120 is actuated, the first
contact 128 provides an electrical connection between the first
input line 126 and the first output line 124. Similarly, the second
cantilever arm 130 is disposed over a second bias substrate
electrode 122. The second cantilever arm 130 also has a second
contact 138 that bridges a gap between the second input line 136
and the second output line 134. When the second cantilever arm 130
is actuated, the second contact 138 provides an electrical
connection between the second input line 136 and the second output
line 134. The switch elements conducting electricity, such as the
first contact 128, the first input line 126, the first output line
124, the first substrate bias electrode, etc., preferably comprise
gold, but other conducting materials such as aluminum, silver,
copper, conducting polymers, etc. may be used.
FIG. 4A shows a close-up view of the first cantilever arm 120 in
the vicinity of the first substrate bias electrode 122 when the
first cantilever arm 120 is in the closed position. As shown in
FIG. 4A, a first portion 129 of the first beam structural layer 116
projects below the top of the first substrate bias electrode 122
between the first substrate bias electrode 122 and the first input
line 126 (not shown) and the first output line 124. FIG. 4A shows
the first portion 129 extending from the first substrate bias
electrode 122 to the first output line 124, but alternative
embodiments according to the present invention have the first
portion 129 not touching the first output line 124 or the first
input line 126. A second portion 127 of the first beam structural
layer 116 projects below the top of the first substrate bias
electrode 122 between the first substrate bias electrode 122 and
the cantilever anchor 117 (not shown). While FIG. 4A shows only the
first portion 129 and the second portion 127 projecting below the
top of the first substrate bias electrode 122, the first beam
structural layer 116 is preferably fabricated such that it
completely surrounds at least a top portion of the first substrate
bias electrode 122 when the first cantilever arm 120 is in the
closed position so that a first substrate bias electrode lid is
provided. That is, it is preferred that a lid is formed in the
first beam structural layer 116 that is defined by the outer
perimeter of the first substrate bias electrode 122.
Returning to FIG. 4, the formation of the preferred lid is further
illustrated by examining the structure of the second cantilever arm
130. As shown in FIG. 4, the second cantilever arm 130 has a first
portion 139 and a second portion 137 of the first beam structural
layer 116, both projecting from the first beam structural layer
116. The area into which the second substrate bias electrode 132
when the second cantilever arm 130 is closed is illustrated by the
recess 135 between the first and second portions 139, 137. Hence,
the recess 135 provides a second substrate bias electrode lid for
the second substrate bias electrode 132. Those skilled in the art
will understand that while FIGS. 4 and 4A show that projected
portions of the first beam structural layer 116 provide the lids
for the first substrate bias electrode 122 and the second substrate
bias electrode 132, other embodiments according to the present
invention may provide the lids with recesses in the first beam
structural layer 116.
In the switch 100 depicted in FIGS. 3, 4 and 4A, the cantilever
anchor 117 becomes a fulcrum to transfer the stress from one side
of the cantilever structure 110 to the other. Thus, a single pole
double throw switch is provided by the two pairs of input and
output lines 126, 124, 136, 134, one pair on each side of the
cantilever anchor 117. A selected input line 126, 136 is closed to
its corresponding output line 124, 134 by actuating the substrate
bias electrode 122, 132 nearest the line, pulling the corresponding
cantilever arm 120, 130 down such that the metal contact 128, 138
makes good contact with the RF lines 126, 124, 136, 134.
Preferably, the lid formed in the first beam structural layer 116
fits snugly around the corresponding substrate bias electrode 122,
132. When the actuation voltage is removed, the friction of the lid
against the corresponding substrate bias electrode 122, 132 keeps
the switch closed. The frictional force may be increased by
fabricating the first beam structural layer 116 so that it also
provides a tight fit between the corresponding substrate bias
electrode 122, 132 and the corresponding input and output lines
126, 124, 136, 134, as shown in FIG. 4A. In this embodiment, the
friction of the lid against the corresponding substrate bias
electrode 122, 132 and the friction of the first beam structural
layer 116 against the corresponding input and output lines 126,
124, 136, 134 will keep the switch closed.
When the other pair of input lines 126, 136 and output lines 124,
134 are to be closed, the cantilever arm 120, 130 on that side is
actuated. By having a slightly flexible cantilever anchor 117, the
stress on cantilever structure 110 from the first side is
transferred to the second side and overcomes the friction forces
holding the cantilever arm 120, 130 on the first side in place.
Thus, cantilever arm 120, 130 on the first side will be released,
while the cantilever arm 120, 130 on the second side will close and
be latched in place.
It is noted that the electrostatic force required to close the
switch depends on the voltage applied to the substrate bias
electrodes 122, 132. In experiments with prior art devices such as
those disclosed by Loo et al., actuation voltages up to 100 V cause
no breakdown in the device. Therefore, it is expected that
embodiments of the present invention may use similar voltages.
Further, a simple current differentiation circuit may provide the
actuation voltage over a relatively short time used to switch the
switch. After that, the control circuits would be shut down until
it was time to switch again. Hence, it can be seen that embodiments
of the present invention do not require a voltage to be constantly
applied to retain the switch in a desired state.
FIGS. 6A-6F illustrate the manufacturing processes embodying the
present invention used to fabricate the switch 100 of FIGS. 3, 4
and 4A. FIGS. 6A-6F present a side profile of the switch 100
similar to that shown in FIG. 4.
The process begins with the substrate 105. In a preferred
embodiment, GaAs is used as the substrate 105. Other materials may
be used, however, such as InP, ceramics, quartz or silicon. The
substrate is chosen primarily based on the technology of the
circuitry the MEMS switch is to be connected to so that the MEMS
switch and the circuit may be fabricated simultaneously. For
example, InP can be used for low noise HEMT MMICS (high electron
mobility transistor monolothic microwave integrated circuits) and
GaAs is typically used for PHEMT (pseudomorphic HEMT) power
MMICS.
FIG. 6A shows a profile of the switch 100 after the first step of
depositing a first metal layer onto the substrate 105 for the first
output line 124 (the first input line 126 is not shown), the first
substrate bias electrode 122, the anchor pad 111, the second
substrate bias electrode 132, and the second output line 134 (the
second input line 136 is not shown) is complete. The metal layer
may be deposited lithographically using standard integrated circuit
fabrication technology, such as resist lift-off or resist
definition and metal etch. In the preferred embodiment, gold (Au)
is used as the primary composition of the first metal layer. Au is
preferred in RF applications because of its low resistivity. In
order to ensure the adhesion of the Au to the substrate, a 900
angstrom layer of gold germanium is deposited, followed by a 100
angstrom layer of nickel, and finally a 1500 angstrom layer of
gold. The thin layer of gold germanium (AuGe) eutectic metal is
deposited to ensure adhesion of the Au by alloying the AuGe into
the semiconductor similar to a standard ohmic metal process for any
III-V MESFET or HEMT.
Next, as shown in FIG. 6B, a support layer 170 is placed on top of
the first metal layer. As can be seen from FIG. 6B, the upper
contour of the support layer 170 generally follows the contour of
the metal layer deposited on the substrate. As discussed in
additional detail below, this facilitates the formation of the
portions 127, 129, 137, 139 of the first beam structural layer used
to latch onto the substrate bias electrodes 122, 132. The support
layer 170 is also etched to the anchor pad 111 to provide for the
formation of the cantilever anchor 117. The support layer 170 may
be comprised of 2 microns of SiO.sub.2, which may be sputter
deposited or deposited using PECVD (plasma enhanced chemical vapor
deposition) or using other techniques known in the art. Etching the
support layer to provide for the formation of the cantilever anchor
117 may be performed using standard resist lithography and etching.
Other materials besides SiO.sub.2 may be used as the support layer
170. The important characteristics of the support layer 170 are a
high etch rate, good thickness uniformity, and conformal coating by
the oxide of the metal already on the substrate 105. The thickness
of the support layer 170 partially determines the thickness of the
switch opening, which affects the voltage necessary to close the
switch as well as the electrical isolation of the switch when the
switch is open. The support layer 170 will be removed in the final
step to release the first and second cantilever arms 120, 130, as
shown in FIG. 6F.
Another advantage of using SiO.sub.2 as the support layer 170 is
that SiO.sub.2 can withstand high temperatures. Other types of
support layers, such as organic polyimides, harden considerably if
exposed to high temperatures. This makes the polyimide sacrificial
layer difficult to later remove. The support layer 170 is exposed
to high temperatures when the silicon nitride for the beam
structural layers 114, 116 is deposited, as a high temperature
deposition is desired when depositing the silicon nitride to give
the silicon nitride a lower HF etch rate.
FIG. 6C shows the fabrication of the first beam structural layer
116. The first beam structural layer 116 is preferably deposited by
PECVD, but other techniques known in the art may be used. The first
beam structural layer 116 is the supporting mechanism of the first
and second cantilever arms 120, 130 and preferably comprises
silicon nitride, although other materials besides silicon nitride
may be used. Silicon nitride is preferred because it can be
deposited so that there is neutral stress in the first beam
structural layer 116. Neutral stress fabrication reduces the bowing
that may occur when the switch is actuated. The material used for
the first beam structural layer 116 should have a low etch rate
compared to the support layer 170 so that the first beam structural
layer 116 (and the second beam structural layer 114) are not etched
away when the support layer 170 is removed to release the first and
second cantilever arms 120, 130.
As shown in FIG. 6C, the first beam structural layer 116 basically
follows the contours of the first metal layer deposited on the
substrate 105. That is, the patterns of the first substrate bias
electrode 122 and the second substrate bias electrode 132 are
transferred to the first beam structural layer 116, due to the
thinness of the first beam structural layer 116. As described
above, this facilitates the latching of the first beam structural
layer 116 to the first substrate bias electrode 122 and the second
substrate bias electrode 132.
After formation, the first beam structural layer 116 is patterned
and etched using standard lithographic and etching processes. Note
that the first beam structural layer 116 is etched after deposit in
the area of the cantilever anchor 117 to provide for the electrical
connection to the anchor pad 111.
FIG. 6D shows the etching of the first beam structural layer 116
used to form dimple receptacles 129, 139. The dimple receptacles
129, 139 are openings where the first contact 128 and second
contact 138 will later be deposited, as shown in FIG. 6E. The
dimple receptacles 129, 139 are created using standard lithography
and a dry etch of the first beam structural layer 116, followed by
a partial etch of the support layer 170. The openings in the first
beam structural layer 116 allow the first contact 128 and second
contact 138 to protrude through the first beam structural layer
116.
Next, as shown in FIG. 6E, a second metal layer is deposited onto
the first beam structural layer 116. The second metal layer forms
the armature electrode layer 112 and the first contact 128 and
second contact 138. In the preferred embodiment, the second metal
layer comprises sputter deposition of a thin film (200 angstroms)
of Ti followed by a 1000 angstrom deposition of Au. The thin film
should be conformal across the switch and acts as a plating plane
for the Au. The plating is done by using metal lithography to open
up the areas of the switch that are to be plated. The Au is
electroplated by electrically contacting the membrane metal on the
edge of a wafer on which the switch (or switches) is fabricated and
placing the metal patterned wafer in a plating solution. The
plating occurs only where the membrane metal is exposed to the
plating solution to complete the electrical circuit and not where
the electrically insulating resist is left on the wafer. After 2
microns of Au is plated, the resist is stripped off of the wafer
and the whole surface is ion milled to remove the membrane metal.
Some Au will also be removed from the top of the plated Au during
the ion milling, but that loss is minimal because the membrane is
only 1200 angstroms thick.
The result of this process is that the armature electrode layer 112
and the first contact 128 and second contact 138 are created in the
second metal layer, primarily Au in the preferred embodiment. In
addition, the Au will fill the area of the cantilever anchor 117
and provide the electrical connection between the anchor pad 111
and the armature electrode layer 112.
After the formation of the armature electrode layer 112 and the
first contact 128 and second contact 138, the second beam
structural layer 112 is deposited. Similar to the first beam
structural layer 116, the second beam structural layer 112 may be
deposited using PECVD, or other techniques known in the art may be
used. The second beam structural layer 112 also preferably
comprises silicon nitride.
It is noted that Au is a preferred choice for the second metal
layer because of its low resistivity. When choosing the metal for
the second metal layer and the material for the beam structural
layers 114, 116, it is important to select the materials such that
the stress in the beam structural layers 116, 117 will not cause
the cantilever arms 120, 130 to bow unacceptably upwards or
downwards when actuating. This is done by carefully determining the
deposition parameters for the structural layers 116, 117. Silicon
nitride is preferred for the structural layers 116, 117 not only
for its insulating characteristics, but, in large part, because of
the controllability of these deposition parameters and the
resultant stress levels of the film.
The beam structural layers 116, 117 may then be further
lithographically defined and etched to complete the switch
fabrication. Finally, the support layer 170 is removed to release
the cantilever arms 120, 130, as shown in FIG. 6F.
If the support layer 170 is comprised of SiO.sub.2, it may be wet
etched away in the final fabrication sequence by using a
hydrofluoric acid (HF) solution. The etch and rinses may be
performed with post-processing in a critical point dryer to help
ensure that the cantilever arms 120, 130 do not come into contact
with the substrate 105 when the support layer 170 is removed. If
contact occurs during this process, unacceptable device sticking
and switch failure may occur. Contact is prevented by transferring
the switch from a liquid phase (e.g. HF) environment to a gaseous
phase (e.g. air) environment not directly, but by introducing a
supercritical phase in between the liquid and gaseous phases. The
sample is etched in HF and rinsed with DI water by dilution, so
that the switch is not removed from a liquid during the process. DI
water is similarly replaced with ethanol. The sample is transferred
to the critical point dryer and the chamber is sealed. High
pressure liquid CO.sub.2 replaces the ethanol in the chamber, so
that there is only CO.sub.2 surrounding the sample. The chamber is
heated so that the CO.sub.2 changes into the supercritical phase.
Pressure is then released so that the CO.sub.2 changes into the
gaseous phase. Now that the sample is surrounded only by gas, it
may be removed from the chamber into room air. A side elevational
view of the switch 100 after the support layer 170 has been removed
is shown in FIG. 6F.
As can be surmised by one skilled in the art, there are many more
configurations of the present invention that may be used other than
the ones presented herein. It is therefore intended that the
foregoing detailed description be regarded as illustrative rather
than limiting and that it be understood that it is the following
claims, including all equivalents, that are intended to define the
scope of this invention.
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